fix(desktop): make model and thinking-level changes instant without success toasts

[amaldevcm]

Summary

Maka Desktop persists active-task model, thinking-level, and permission-mode
changes through Runtime Host asynchronously. Model and thinking-level writes
shared a single boolean pending gate, so a thinking-level change could be
blocked by an in-flight model change (and vice versa); every completed change
also fired a success toast regardless of whether it was still the latest
intent.

This replaces the shared gate with an independent optimistic-value overlay per
session for each of model, thinking-level, and permission-mode. A change
applies to the UI immediately; the overlay value itself is the coalescing
token, so a stale completion (superseded by a newer change to the same field)
is silently dropped instead of toasting or rolling back over a newer
selection. Success is now silent; only a terminal failure (the latest
request failing) rolls the control back to the last committed value and shows
the existing localized error toast.

Fixes #3745

Verification

• tsc --noEmit clean on every changed file (apps/desktop/tsconfig.main.json
  and tsconfig.renderer.json)
• Rewrote app-shell-session-settings-actions.test.ts (12 tests: latest-wins
  coalescing, rollback-on-failure, a superseded failure not rolling back a
  newer selection, model/thinking independence, explicit-undefined
  thinking-level override) and fixed app-shell-session-ui-state.test.ts's
  fixtures for the renamed state fields — 27/27 pass
• npm run lint (biome) clean on all changed files
• Full apps/desktop main test suite: 1289/1315 pass; the 10 pre-existing
  failures (OAuth deadline timing, Windows symlink privileges, Computer Use
  health, RiveWorkflow) are unrelated — confirmed identical with these changes
  stashed out
• format:check and the root build/typecheck scripts were not run
  end-to-end: packages/ui's build currently fails on pre-existing, unrelated
  TypeScript errors (missing lucide-react/simple-icons type declarations,
  a settledText API drift) — verified this predates this change by
  reproducing it with the diff stashed out
• Not done: manual click-through in the running desktop app (rapid
  thinking-level changes, forced-failure rollback, bypass-confirm gating)

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Claude (Sonnet 5) — investigated the issue, designed and
implemented the full fix across all files listed below, and wrote/updated the
tests. Generated-by: Claude trailer is on the commit.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally (format
  and the full-workspace typecheck/build were not run — see Verification)

Does this PR entail a change in behavior?

• Yes — described under Summary above

[amaldevcm]

Closing — #3749 already exists for this issue, opened by the person who claimed it first (Sun-GLiang), and has gone through several rounds of review that surfaced races and edge cases (Stop cancellation, cross-client staleness, follow-up settlement) this implementation didn't handle. Deferring to that PR instead of duplicating the effort.